The proposed research is planned within two categories. The first will analyse the effects of hippocampal damage in adult monkeys, while the second will be concerned with the effects of equivalent lesions in rats as function of age, enriched environment and sex. Experiments in the first category will attempt to: 1. Find out whether retention deficits after hippocampal resections are due to impaired recognition or associative learning. To this end, the effects of fornix and hippocampal damage will be compared on matching-to-sample tasks. 2. Determine whether motivational effects of fornix and hippocampal lesions are functionally dissociable: The performance of monkeys with fornix and hippocampal lesions will be compared on a visual novelty-seeking task. In the second category, we intend to determine whether early enriched environment can offset the deleterious effects of early and late hippocampal ablations. Rats will be reared in different environments, perinatal lesions induced at different stages of hippocampal structural and functional maturation and performance assessed on spatial and nonspatial tasks. Experimental neuroanatomical methods will be used in the search for morphological correlates of early fornix or entorhinal ablations. Plasticity of the limbic system will be studied by examining the recovery rate after early injury.